In order to cope with scale expansion or frequency increase of landslide disasters or flood disasters due to frequent occurrence of abnormal weather in recent years, issuance of heavy rainfall and flood warning or advisory, prediction of risk of a landslide, and the like are conducted by the Meteorological Agency, using a soil water index and a basin water index, which are considered to be good indices for such disasters.
The soil water index is an index into which quantity indicating how much fallen rain has been retained inside soil (hereinafter, simply referred to as “in soil”) is transformed based on rainfall amount data, using a tank model. In the above, the tank model models a process in which fallen rain flows on a surface of the ground into a river or seeps into the ground by likening the process of tanks having some outlets, as illustrated in FIG. 1.
When using the tank model, in general, a surface of the ground is divided into 5 km square lattice (mesh) elements and calculation is performed for each lattice element using tanks stacked in three stages. On a side face of each of the tanks stacked in three stages, a runoff outlet which represents that water runs off to the surrounding soil is formed, and on the bottom face thereof, a seepage runoff outlet which represents that water seeps into a deeper portion is formed. Runoff volume from the runoff outlet on the side face of the first tank corresponds to surface runoff, runoff volume from the runoff outlet on the side face of the second tank corresponds to seepage runoff at a surface layer, and runoff volume from the runoff outlet on the side face of the third tank corresponds to runoff as groundwater. In addition, inflow to the first tank corresponds to precipitation, inflow to the second tank corresponds to runoff from the seepage runoff outlet of the first tank, and inflow to the third tank corresponds to runoff from the seepage runoff outlet of the second tank. The soil water index represents the total moisture content (the amount of storage) remaining in the respective tanks and corresponds to moisture content in soil.
A landslide disaster such as a debris flow and a landslide caused by heavy rain has a higher occurrence probability as moisture content in soil increases, and there is a case where rain that fell many days earlier may influence an occurrence of a landslide disaster. The soil water index is used for a criterion for issuance of landslide alert information and heavy rain warning or advisory, which meteorological observatories in various regions issue, as an index representing increase in risk of occurrence of a landslide disaster because of heavy rain.
PTL 1 discloses a disaster prediction system and a disaster prediction method using the soil water index. The system disclosed in PTL 1 sets an inclination direction and an inclination angle based on terrain data for each preset fixed section, and determines a surface runoff coefficient that is a coefficient relating to surface runoff of water on the ground surface and a soil runoff coefficient that is a coefficient relating to seepage runoff in the ground, based on the inclination direction and the inclination angle. The system calculates a degree of risk for each section at every unit time based on the soil water index, the surface runoff coefficient, and the soil runoff coefficient for each section, and displays the degree of risk in the section.
NPL 1 describes a calculation method of a basin water index. The basin water index is an index how much degree rainwater having fallen in the basin of a river influences downstream areas, obtained based on the amount of rain fallen in the past (radar-raingauge analyzed precipitation) and the amount of rain forecast to fall within several hours thereafter (short-term precipitation forecast), by calculating of a runoff process and a flow down process. In the method described in NPL 1, a surface of the ground is partitioned into 5 km square sections in a runoff process, and a process in which rain having fallen in each section runs off to a river is calculated using a tank model.
The method described in NPL 1 takes a fact into consideration that most fallen rain flows on the surface of the ground in an urban area in which the ground surface is covered with concrete, while fallen rain seeps into the ground to become groundwater or flows on the surface of the ground and flows into a river in general. In particular, the method performs calculation using a single stage tank model for an urban area, which mainly deals with surface runoff, and a three-stage series tank model for a non-urban area. Subsequently, in a flow down process, flow of rainwater is calculated, with respect to the rainwater of which the inflow amount into a river is calculated using the runoff process. In the method, in order to calculate the flow of the rainwater in the flow down process, a river channel in a 5-km lattice element is partitioned into six areas along the course of the river, and temporal fluctuation in the amount of flowing-down rainwater is calculated.
A flood disaster (such as swelling and flooding of a river) caused by heavy rain has a higher occurrence probability as the amount of flowing-down rainwater increases, and it is required to consider a time lag before rain having fallen upstream converges downstream. The basin water index is, as an index devised with the above requirements taken into consideration, used for a criterion for issuance of flood warning and advisory which meteorological observatories in various regions issue.
Techniques for estimating moisture content at the ground surface over a wide area include a technique described in NPL 2. In addition, NPL 3 describes a technique for calculating a degree of risk at which a landslide disaster may occur from moisture content in soil.